It is typical for airborne navigation to aid alignment of an Inertial Measurement Unit (IMU) or Inertial Navigation System (INS) with Global Positioning System (GPS) information. For straight and level flight, this works well except in one respect. While 3-dimensional accelerations can be aligned, as well as angular rotation offsets about horizontal axes (e.g. pitch and roll for a level platform), the third angular rotation axis, i.e. about the vertical axis, remains problematic. In straight and level flight, without horizontal decoupled accelerations, the orientation about the vertical axis is not observable in GPS derived measurements.
This rotational error about the vertical axis is sometimes referred to as ‘yaw’, and sometimes as ‘heading error’. It is crucial to distinguish heading error as an error in the orientation of the IMU, and not as an error in the direction of translation of the platform. These are two separate things that are often confused.
It is important to appreciate that the geolocation capability and focusing of a SAR image (i.e. the pointing of the synthetic antenna) depend on the translational motion measurement accuracy, whereas the pointing of the real antenna depends on the rotational orientation motion measurement accuracy.
To first order, an excessive heading error will cause the antenna to be mis-pointed and the desired scene to be improperly illuminated by the antenna, or perhaps even not at all. The SAR image will exhibit improper reflectivity measurements, likely with a brightness gradient across the image. This has a deleterious effect on Signal-to-Noise Ratio (SNR) thereby reducing image quality, and target feature detectability. In multi-phase-center antennas, any Direction of Arrival (DOA) measurements may be unacceptably impaired.
The conventional treatment for reducing heading error in a SAR system is for the platform occasionally to experience substantial lateral accelerations via dedicated maneuvers to make the heading error observable, and hence correctable. One such maneuver is the notorious S-turn. Such maneuvers are often an interruption to the platform's mission, and generally undesirable to aircraft operators and crew. Depending on the quality of the IMU and the SAR image requirements, such maneuvers might be necessary every few minutes.
Alternatively, additional motion measurement instrumentation may be employed that makes heading errors observable, e.g. a second IMU with a transfer alignment. This is generally expensive and thereby usually nonviable as a solution. Sometimes it is nonviable regardless of cost, due perhaps to space limitations, limited communication channels, or lack of availability of the second IMU.
It is desirable in view of the foregoing to provide for heading error corrections without the aforementioned difficulties associated with conventional approaches.